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Photon detector is precursor to broadband in space

By Kimm Groshong

US researchers have nearly trebled the efficiency of a miniscule detector capable of capturing single photons of light – the technology could one day be used to receive information through a laser stream of data sent from Mars to Earth. The finding could lead to speedier, reliable relays of huge amounts of data across interplanetary distances, setting up a form of broadband communication in space.

“It can take hours with the existing wireless radio frequency technology to get useful scientific information back from Mars to Earth. But an optical link can do that thousands of times faster,” says Karl Berggren, at MIT.

His team has boosted the photon-capturing abilities of the detector by using an extremely thin nanowire detector. They combined it with an anti-reflection coating to stop light bouncing away and a “photon trap” that helps channel incoming photons to be absorbed and not lost.

The trap – a cavity between a sheet of glass and a gold mirror at a set distance – reflects photons that would normally transmit straight through the detector back onto the coiled nanowire, where they can be absorbed.

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Superconductor

The special add-ons increased the detector’s absorption efficiency from 20% – the previous best for previous single-photon detectors – to 57% at the wavelength used for broadband signal transmission.

And it does so quickly. “The speed comes from the fact that it’s a superconducting nanowire,” Berggren says. The nanowire is wrapped tightly into a configuration with dimensions many times smaller than a human hair. It is cooled to just above absolute zero, where it becomes a superconductor sensitive to absorbed photons.

The speed and efficiency of detection would be crucial in interplanetary optical communications. Unlike traditional radio links, clouds become a problem in the transmission of photons. And a laser beam would spread out many times by the time it reached the Earth, making the received signal very weak. So the efficiency of detecting light particles is vital in developing any kind of optical communication.

“They can work with very weak signals and very few photons,” Andrew Kerman, a member of the technical staff at Lincoln Laboratory in Lexington, Massachusetts, US, told New Scientist. “In communicating with Mars, for example, you’re going to be stuck not being able to transmit very many photons.”

Quantum cryptography

When NASA cancelled its Mars Telecommunications Orbiter in July 2005, a team of researchers had been developing an optical communication laser for just such a purpose. The laser was designed to beam back between 1 million and 30 million bits per second, depending on the distance between Mars and Earth at any given time.

The currently orbiting Mars Odyssey spacecraft relays about 128,000 bits per second using radio waves. The leap in capacity offered by lasers is due to the different wavelengths of light carrying the data. The Mars Telecommunications Orbiter was to use infrared light with a wavelength of 1.06 microns – thousands of times shorter than radio waves. Light and radio travel at the same speed through space, but shorter wavelengths carry more information in the same transmission time.

But interplanetary communication is not the only application for such an efficient single-photon detector. Berggren also foresees applications in quantum cryptography where the state of individual photons encodes information that needs to be secure.